Suppose \(L\)  is an operator with a compact resolvent and
\(s_{n}(R_{L})\le C \,n^{-\mu },\,n\in \mathbb {N},\,0\le \mu < \infty ;\)  then
we
denote by \(\mu (L) \)  order of the operator \(L\)  in accordance with the definition given in the paper [1]}.
Denote by \( \mathfrak {Re} L := \left(L+L^{*}\right)/2,\, \mathfrak {Im} L := \left(L-L^{*}\right)/2 i\) 
the real and imaginary components of an operator \(L\)  respectively.
In accordance with the terminology of the monograph [2]} the set \(\Theta (L):=\lbrace z\in \mathbb {C}: z=(Lf,f)_{\mathfrak {H}},\,f\in \mathrm {D} (L),\,\Vert f\Vert _{\mathfrak {H}}=1\rbrace \)  is called the numerical range of an operator \(L.\) 
An operator \(L\)  is called sectorial if its numerical range belongs to a closed
sector \(\mathfrak { L}_{\gamma }(\theta ):=\lbrace \zeta :\,|\arg (\zeta -\gamma )|\le \theta <\pi /2\rbrace  ,\)  where \(\gamma \)  is the vertex and \( \theta \)  is the semi-angle of the sector \(\mathfrak { L}_{\gamma }(\theta ).\) 
An operator \(L\)  is called bounded from below if the following relation holds \(\mathrm {Re}(Lf,f)_{\mathfrak {H}}\ge \gamma _{L}\Vert f\Vert ^{2}_{\mathfrak {H}},\,f\in \mathrm {D} (L),\,\gamma _{L}\in \mathbb {R},\)  where \(\gamma _{L}\)  is called a lower bound of \(L.\)  An operator \(L\)  is called accretive if \(\gamma _{L}=0.\) 
An operator \(L\)  is called strictly accretive if \(\gamma _{L}>0.\)  An operator \(L\)  is called m-accretive if the next relation holds \((A+\zeta )^{-1}\in \mathcal {B}(\mathfrak {H}),\,\Vert (A+\zeta )^{-1}\Vert  \le (\mathrm {Re}\zeta )^{-1},\,\mathrm {Re}\zeta >0. \) 
An operator \(L\)  is called m-sectorial if \(L\)  is sectorial and \(L+ \beta \)  is m-accretive for some constant \(\beta .\)  An operator \(L\)  is called symmetric if one is densely defined and the following equality holds \((Lf,g)_{\mathfrak {H}}=(f,Lg)_{\mathfrak {H}},\,f,g\in \mathrm {D} (L).\)
